work around rodin db exception when a symbolic attribute is not set

de.prob.symbolic/src/de/prob/symbolic/SymbolicAttribute.java

@@ -38,8 +38,14 @@ public class SymbolicAttribute extends AbstractBooleanManipulation {
 	@Override
 	public String getValue(IRodinElement element, IProgressMonitor monitor)
 			throws RodinDBException {
+		try {
 			return asInternalElement(element).getAttributeValue(ATTRIBUTE) ? SYMBOLIC
 					: CONCRETE;
+		} catch (RodinDBException ex) {
+			// happens if the attribute is not set on this element
+			// just return a default instead of throwing a RodinDBException
+		}
+		return CONCRETE;
 	}
 
 	@Override